Venturi multiphase flow measurement based active slug control

Riser slug flow poses a significant challenge to offshore oil production systems, most especially for oil fields in their later life. Active control of slugging through choking has been proven a practical approach in eliminating riser slug flow in oil production pipeline-riser systems. However, existing conventional active slug control systems may reduce oil production significantly due to excessive over choking. Again, some of the existing active slug flow control systems rely on seabed measurements, which are difficult to maintain, costly to install, unreliable, and seldom readily available. This study is an experimental investigation of the feasibility of active riser slug control by taking topside differential pressure measurement from the inlet of the venturi flow meter to the throat. Experimental results indicate that under active slug flow control, the system was able to eliminate slug flow at a higher valve opening when compared to manual choking. A valve opening of 24% with riser base pressure at 2.85 bar from open loop unstable of 23% was recorded, which is superior to manual choking which maintained flow stability up to 21% valve opening with riser base pressure of 3.8 bar

Slug flow control in an S-shape pipeline-riser system using an ultrasonic sensor

Slugging flow poses significant challenges to the offshore multiphase flowline and riser systems. Slug flow is characterized by an uneven flow regime whereby pipeline pressures, temperatures, or flow volume rates fluctuate. One of the most common causes of severe slugging is low pressures which causes buildup of fluid over time, consequentially causes flow and pressure oscillations. This mostly occurs in vertical risers or wells. The negative effects of severe slugging have prompted numerous studies, investments, and efforts to reduce or eliminate the slugging flow. Several active slugging control techniques have been investigated in the oil and gas industries for decades. However, many of these techniques still run the risk of limiting hydrocarbon production due to inappropriate over choking. Other challenges for active slug control include the fact that some systems rely mainly on subsea measurements such as riser base pressure, and most of these subsea measurements are costly, difficult to maintain, not always available, and can be unreliable. As a result, to achieve an efficient slugging control performance, reliable, robust, and efficient measurements that are more sensitive to slugging flow for control are required, which is the motivation for this work. The control of riser slug flow using non-radioactive, non-invasive, and non-intrusive Continuous-wave Doppler ultrasound has been investigated in this work, and provides good control performance. It achieved a larger valve opening than an open-loop unstable system. This outperforms manual choking, which maintains stability at a much lower valve opening

Slug flow control using topside measurements: a review

Slugging flow is a condition caused by a liquid obstruction at the riser base. It exhibits cyclic behaviour. The cycle consists of a protracted time of no gas production at the riser's top, followed by the arrival of a liquid slug with a length greater than the riser height, and ultimately the breakthrough of a significant gas surge. The cycle time might range from a few minutes to a few hours, depending on the system size and flow conditions. In offshore oil production, feedback control is a practical and cost-effective way to prevent slug flow. To control the flow rate or the pressure in the pipeline, adjusting the choke valve opening on the topside facility is generally utilised as the control input. From a practical standpoint, designing a control system based on topside data rather than seabed measurements is preferable. Controlling the topside pressure alone is difficult and ineffective in reality, but combining it with the flow rate results in a more reliable control solution. Measuring the flow rate of a multiphase flow, on the other hand, is difficult and expensive. All the topside measurements-based slug control techniques was critically reviewed and necessary recommendations for enhanced control performance provided. In conclusion, this review acknowledged that slugging is a well-defined flow pattern, yet despite having been studied for several decades, current slug control methods still have robustness issues. Slug flow problems are expected to become even more intense in the future as a result of longer vertical risers driven by deep-water Exploration and Production (E&P)

Multiphase flow instability and active slug control solutions.

Slugging as a flow assurance challenge is an upsetting condition to the oil and gas industry due to the instabilities it poses on the system. The negative repercussions associated with slug flow stems from the inlet through to the topside facilities where processing is done. Active control has been established as one of the best techniques to eradicate slug and its accompanying challenges however the controller robustness and some setbacks make improvement a necessity. In that vein, the Inferential slug controller which uses a combination of topside measurement signals to produce a single control variable which is more sensitive to slug variations hence can effectively be used to control slug, was invented. Again the robustness of this controller has been in question. This study presents a comprehensive and systematic analysis of the Inferential slug controller design for system stability analysis and maximising throughput from unstable riser pipeline system configurations in the quest to advance this technology. The inferential slug controller’s robustness was assessed by implementing this technique on several pipeline riser systems including U-shape and S-shape riser configurations. Prior to that, the flow behaviour for a wide range of flow conditions was investigated, highlighting the impact of geometry on unstable slug flow through the OLGA flow simulator (modelling) and experiments. New and unused measurement signals from the topside of either the riser/platforms were deployed in the inferential slug control technology to make the controller more sensitive and robust. A simplistic nevertheless robust procedure for designing the inferential slug controller was proposed. Unstable slug flow conditions were observed to stabilise at a relatively larger valve opening compared with that seen in open loop. The inferential slug controller technology is further extended to deal with systems with variable time delay using a proposed modified Smith predictor model. The modified Smith predictor was recorded to improve and stabilise a pipeline riser system which has deteriorated in control performance due to time delay in the system, a resultant of large stroke time in the valve. This in practicality means an increased production through the system. In advancing the ISC technology to be deployed on offshore fields in conjunction with other passive slug mitigation techniques, the slug mitigation potential of pseudo spiral tube (PST) was assessed when installed at the topside of the riser system. The analysis showed that the PST pipe section (spiral and wavy piece) when installed at the topside of the riser system, possesses some mitigation potential. Four different slug regions was identified for the entire pipeline system. The first region being a slug flow occurrence in the system with and without the PST whiles the second region is the region where slugging occurs in the system but disappears when coupled with the PST and the opposite describes the third region. Lastly, the fourth region is described as that region where slugging flow exist for the system coupled with the spiral pipe section and without any PST (plain) but slugging flow disappears when the system is coupled with the wavy pipe section. The wavy or spiral pipe section coupled with the S-shape riser system have slug mitigation capabilities when they are installed at the top of the riser although its effectiveness of slug mitigation depends on the flow condition. This is evident in the significant reduction in the riserbase pressure oscillation magnitude and the significant reduction in the slug envelope (region) when the system was coupled with the wavy or spiral pipe section relative to the plain system.PhD in Energy and Powe

New multiphase flow measurements for slug control.

Severe slug flow is undesirable in offshore oil production systems, particularly for late-life fields. Active control through choking is one of the effective approaches to mitigating/controlling severe slug flow in oil production pipeline-riser systems. However, existing active slug control systems may limit oil production due to overchoking. Another problem in most active control systems is their dependency on information obtained from subsea measurements such as riser base pressure for active slug flow control. Both of these control challenges have been satisfactorily solved through the introduction of new multiphase flow topside measurements that are reliable and efficient in providing flow information for active slug control systems. By using Venturi multiphase flow topside measurements and Doppler ultrasonic measurements, an active slug flow control system is proposed to suppress severe slug flows without limiting oil production. Experimental and simulated results demonstrate that under active slug control, the proposed system is able not only to suppress slug flow but also to increase oil production compared to manual choking. Another objective of this research was to assess the applicability of continuous-wave Doppler ultrasonic (CWDU) techniques for accurate identification of gas-liquid flow regimes in pipeline-riser systems. Firstly, flow regime classification using the kernel multi-class support-vector machine (SVM) approach from machine learning (ML) was investigated. For a successful industrial application of this approach, the feasibility of conducting principal component analysis (PCA) for visualising the information from intrinsic flow regime features in two-dimensional space was also investigated. The classifier attained 84.6% accuracy on test samples and 85.7% accuracy on training samples. This approach showed the success of the CWDU, PCA-SVM, and virtual flow regime maps for objective two-phase flow regime classification on pipeline-riser systems, which would be possible for industrial application. Secondly, an approach that classifies the flow regime by means of a neural network operating on extracted features from the flow’s ultrasonic signals using either discrete wavelet transform (DWT) or power spectral density (PSD) was proposed. Using the PSD features, the neural network classifier misclassified 3 out of 31 test datasets and gave 90.3% accuracy, while only one dataset was misclassified with the DWT features, yielding an accuracy of 95.8%, thereby showing the superiority of the DWT in feature extraction of flow regime classification. This approach demonstrates the employment of a neural network and DWT for flow regime identification in industrial applications, using CWDU. The scheme has significant advantages over other techniques in that it uses a non-radioactive and non-intrusive sensor. The two investigated methods for gas-liquid two-phase flow regime identification appear to be the first known successful attempts to objectively identify gas-liquid flow regimes in an S-shape riser using CWDU. The CWDU approaches for flow regime classification on pipeline-riser systems were successful and proved possible in industrial applications.PhD in Energy and Powe